Among various light-emitting devices, organic electroluminescent (EL) devices have been actively investigated and developed in recent years, since they can emit luminescence of high luminance at a low driving voltage. In general, the organic electrolight-emitting device is composed of a light-emitting layer and a pair of counter electrodes between which the light-emitting layer is interposed, and electrons injected from the cathode and holes injected from the anode are recombined in the light-emitting layer to generate excitons luminescence from which is utilized.
At present, an organic electrolight-emitting device which emits luminescence of high luminance at a low driving voltage has a stratified structure as described in Tang et al, Applied Physics Letters, Vol. 51, page 913 (1987). The organic electrolight-emitting device can emit green luminescence of high luminance by laminating an electron-transporting and luminescent material and a hole-transporting material, and the luminance reaches to several thousands of candelas per square meter at a direct current voltage of 6 to 7 V. However, from the standpoint of practical use, it has been desired to develop a light-emitting device having higher luminance and higher luminous efficiency.
Recently, in order to develop light-emitting devices having higher luminous efficiency, light-emitting devices employing various transition metal complexes as luminescent materials have been investigated. As an light-emitting device which emits in particularly high luminous efficiency, an light-emitting device using an ortho-metalated complex of iridium (tris-orthoiridated complex with 2-phenylpyridine (Ir(ppy)3)) as a luminescent material is reported in Applied Physics Letters, Vol. 75, page 4 (1999). The light-emitting device has an external quantum yield of 8.3%. This value exceeds an external quantum yield of 5% which has been regarded as the limit value. However, since it is restricted to a green light-emitting device, development of other color light-emitting devices having higher luminous efficiency is necessary in order to apply to a full-color display or a white light-emitting device.
On the other hand, the organic light-emitting devices which provide luminescence of high luminance are those having a stratified structure of organic substance formed by vacuum deposition. While the production of devices using a coating technique is desirable from the standpoints of simplification of production process, processability, increase in size, etc., the devices produced using any conventional coating technique are inferior to the devices produced by using vacuum deposition in view of the luminance and luminous efficiency. Accordingly, an important subject has been to increase the luminance and luminous efficiency.